In a number of industrial operations, it is necessary to have a cable, hose, or other flexible member (hereinafter collectively referred to as a "cable") extending from a fixed terminus to a moving body, where the cable so extending must be taken up and maintained in a taut condition between the moving body and the fixed terminus.
One operation where this requirement is well-known is underground mining, where electrically-operated trackless vehicles and mining machines are becoming increasingly common. Such vehicles are typically connected to a main electrical supply via a cable extending from the vehicle to one of a number of regularly-spaced plug-in power sources, termed tie points. In order to prevent entangling the cable beneath the wheels of the vehicle or on obstacles while the vehicle is moving about, it is necessary to keep excess cable coiled on a reel or analogous device from which cable can be either payed out or payed in as the vehicle moves away from or toward the tie point, respectively. Usually, a number of tie points regularly spaced apart are available in the mine, thereby obviating the need for extremely long cables. Moving the vehicle long distances in the mine requires that the cable be unplugged from a more distant tie point and reconnected to a nearer tie point. Usually, mining machines are employed for periods of time in localized areas in the mine near a single tie point, where the tie point is located somewhere near the expected midpoint of vehicular travel at that location. Because there are many such power sources in a mine, economic considerations dictate that the take-up reel for the cable be mounted on the mining vehicle, not at each tie point. However, in other industrial applications it may be preferable to have the reel secured to a fixed location and the cable extending therefrom to a moving body or vehicle.
Whether the reel is located on the vehicle or is stationary, the mechanism that drives the reel must satisfy certain requirements. First, excess length of the cable must be kept wound upon the reel, preferably in an ordered lapped fashion to prevent kinking and damage to the cable. Second, the cable extending from the vehicle to the tie point must be maintained in a taut condition at all times. In other words, the reel must be driven to tautly pay in cable whenever the vehicle is moving toward the tie point, to keep the cable taut between the vehicle and tie point whenever the vehicle is remaining stationary, and to tautly pay out cable whenever the vehicle is moving away from the tie point. Third, all such motions of the reel should occur automatically. Fourth, the reel-like mechanism should be powerful to accommodate a heavy cable on a large, heavy reel. For example, an electrical cable for a modern mining vehicle can be three inches or more in diameter and weigh several tons when five hundred feet of it is fully coiled. A reel for such a cable can be up to five feet in diameter and five feet wide. Fifth, as experience has shown that hydraulically-driven reel mechanisms offer the best combination of power, quickness of response, and reliability, such hydraulic mechanisms must be capable of performing expected functions without excess heat generation. Such excess heat is a common problem with mobile hydraulic mining machinery constructed within difficult size and space constraints. Excess heat can cause premature failure of the hydraulic fluid (usually oil) and the hydraulic equipment. Improperly designed hydraulic equipment intended for high-power applications can experience catastrophic heat generation in a remarkably short period of time.
Several cable reel mechanisms have heretofore been patented. Slomer (U.S. Pat. No. 2,665,081) discloses a hydraulic controller intended for use with a positive-displacement, constant-output hydraulic pump, a hydraulic motor driven by the pump, and a reel driven by the motor. The controller includes an adjustable relief valve responsive to a change in the direction of hydraulic flow to vent excessive hydraulic pressure from the pump downstream to a fluid reservoir whenever the reel is stalled (vehicle stationary) or paying out cable (vehicle moving away from the power source). The only time the pump is not pumping fluid through the relief valve is when the reel is paying in cable at top speed. As a result, the Slomer device has serious disadvantages. In particular, because the reel motor is driven with a fixed-displacement pump, hydraulic fluid from the pump when not being used to drive the hydraulic motor must be externally shunted back to the fluid reservoir. Otherwise, the pump will stall or cause rupture of hydraulic conduits due to excessive pressure buildup. Unfortunately, such external shunting of hydraulic fluid from the pump causes rapid heating of the oil, especially at higher operating pressures. As a result, the Slomer device is only operable up to approximately 300 psig hydraulic pressure without overheating, which is many times less than operating pressures required on many types of modern equipment.
Maier (U.S. Pat. No. 4,114,827) discloses a hydraulic controller for a cable reel powered by a hydraulic motor supplied by a fixed-displacement, continuous-output pump. The controller includes a piloted and spring-biased bypass valve for unloading the pump whenever the vehicle is stationary or whenever cable is being payed out. The bypass valve pilot senses hydraulic pressure downstream of the reel motor, such pressure increasing during cable pay-out and when the reel is stalled. The controller also includes a pressure relief valve to ensure that pressure downstream of the pump never exceeds a preset limit (450 psig). Similar to the Slomer device, however, the Maier controller is designed for use with a fixed-displacement, constant-output hydraulic pump. As a result, excess pressure downstream of the pump during reel stall or cable pay-out must be externally shunted to the reservoir. Such shunting causes excessive heating of the hydraulic fluid unless the hydraulic pressure is maintained below a limit (450 psig) too low for many modern applications.
Carlson (U.S. Pat. No. 3,334,839) discloses another hydraulic system utilizing three fixed-displacement pumps: one to act as a reel-motion sensor, and two others to supply hydraulic pressure to the reel motor. The reel-motor pressure is controlled by a remote controlled relief valve that, in turn, is controlled by a system of shuttle valves. This system is less prone to heating than the Slomer and Maier systems, but is still limited only to about 450 psig or below. Further, the system is relatively complex with three fixed-displacement pumps instead of one, which decreases reliability. Finally, the 450 psig pressure limit is too low for many modern requirements.
The Lee patent (U.S. Pat. No. 2,912,184) discloses a hydraulic system for an entire mine haulage vehicle including its reel drive system. Two hydraulic pumps are employed to drive the reel, the first powered by an electric motor and the second by the vehicle's motion. The motor-driven pump is used for conditions requiring a relatively low hydraulic fluid flow rate, and the vehicle-driven pump is used for high-flow conditions. A disadvantage of this system is that the pumps are fixed-displacement types, which require that the pressures be kept low to avoid excessive heating of the system.
Oetringhaus (U.S. Pat. No. 4,511,100) and Pollman et al. (U.S. Pat. No. 4,537,364) disclose different types of closed-loop hydrostatic transmissions for cable reel drive systems that are controlled by electro-hydraulic servomechanisms. Since the hydraulic pumps are variable-displacement and bidirectional, there is no need to pump hydraulic fluid over relief at any time. As a result, both systems are capable of operating at pressures higher than 450 psig. Unfortunately, however, the systems incorporate a relatively large number of both electrical and mechanical components which render the systems complicated, expensive and less reliable than simpler all-hydraulic systems.
Modern, efficient underground mining operations, especially hard-rock mining but also including large-scale soft-rock (e.g., coal) mining, require much larger and more powerful vehicles and other equipment than in the past. Correspondingly, hydraulic systems on such vehicles and equipment must be much larger and more powerful than in the past. Where prior-art systems as described above operated satisfactorily at their pressure limits of 300-450 psig, modern systems require pressures of 2,000-3,000 psig or higher for satisfactory performance, which levels are simply beyond the capability of many prior-art systems. Further, since underground mining equipment must be reliable and maintenance services in most mines are irregular at best, relatively simple all-hydraulic systems not requiring electro-mechanical or other sophisticated sensors and feedback circuits have proven to be superior in terms of reliability and serviceability.
Hence, there is a need for a hydraulically powered reel mechanism capable of operating at 2,000 psig hydraulic pressure or higher without experiencing destructive heat buildup.
There is also a need for such a system operable only via, and responsive entirely to, hydraulic pressure for minimal complexity and maximal reliability.
There is also a need for such a system sufficiently powerful to operate the large heavy reels and cables required with modern industrial machinery.